Recent successes in the field of gene therapy have highlighted the feasibility and efficacy of gene transfer approaches in treating numerous diseases.(1-3) While these achievements are encouraging, many significant hurdles remain before we can hope to reap the full benefits of applying this technique more generally. Age-related macular degeneration (AMD), an incurable disease responsible for an estimated 1.75 million cases of blindness in the aging population of the United States alone,(4) represents a huge public health burden for our population of older Americans. An important feature of AMD is the pathologic degeneration of the photoreceptors, which would otherwise serve as the primary target for many AMD therapeutics.(7-8) This common pathology may also provide an opportunity to broadly address all forms of AMD with a single therapeutic approach. By expressing the protein Channelrhodopsin-2 (ChR2) in the bipolar cells of the retina, each bipolar cell could be transformed into its own photoreceptor, circumventing the degeneration of rods and cones, providing a universal cure for AMD.(15-17) While the feasibility of this approach has been shown in small animal models, the resistance of the bipolar cells to AAV transduction remains to be addressed.(18) Successful retargeting of AAV to difficult-to-transduce cell types by modifying its interactions with cell surface receptors has been shown in vitro and in vivo.(33-38) A recombinant diabody (scDb), i.e., an antibody-derived molecule capable of binding two different epitopes, designed to bind both AAV and a receptor preferentially expressed on the bipolar cells, could be employed to achieve such retargeting. We propose to produce such a scDb and demonstrate its ability to retarget AAV in vitro. The further development of this system could allow for specific retargeting of AAV to the retinal bipolar cells. [The immune response to retinal expression of ChR2, an algal-derived transgene, will be critical to the further development of this technique. To investigate this, we propose, as our second aim, a study to thoroughly examine local toxicity, systemic immune response, and transgene expression for high, medium, and low doses of AAV2/8TC-ChR2 (a novel AAV serotype we have developed with our collaborators that more efficiently targets retinal bipolar cells) administered subretinaly in mice. This study is designed to characterize the difference in immunogenicity and expression pattern between ChR2 expression driven by either the constitutive CAG promoter or the ON bipolar cell specific mGluR6 promoter. Additionally, should the scDb described in Aim 1 be produced and validated, Aim 2 is designed to accommodate the inclusion of this scDb in a separate animal cohort for in vivo validation and testing. Thus, this study will not only provide important information about the immune response to ChR2 and its expression in the retina, but will also inform any future ChR2-based gene therapy studies.] In pursuing these aims, we hope to address two major obstacles regarding ChR2 based ocular gene therapy - delivery and immunogenicity - and in doing so to specifically advance the field of gene therapy for AMD, and more generally to lay the foundation for effective scDb-based AAV retargeting for other diseases.